drm/amd/powerplay: add interface to get clock by type with latency for display (v2)

drm/amd/powerplay: add interface to get clock by type with latency for display (v2)

This patch adds get clock by type with latency, display will use it to get
current clocks with latency.

v2: fix the missed mutex lock before return.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>